Porting and Flow Testing to Increase Horsepower

As I have established, high-airflow potential, especially through the cylinder head, is a key element in producing power. If you are starting with as-cast parts, a means of reshaping them and testing the subsequent flow results are very useful aids to maximizing output. In short, the subjects in this chapter are physically porting heads, intakes, etc.; what equipment you might need; and the results you may reasonably expect.

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So far we have talked about flow and flow bench results. Let’s be clear about one thing: It is entirely possible to exist without a flow bench to verify what you have done. But doing really good porting work is in the details, not the overall. Sometimes what seems like an obvious move to increase flow does the opposite, and what could possibly have been a big overall improvement is reduced to something a little less. Two or three mistakes like this may mean the difference between winning and being a runner-up. A flow bench is not only a real aid to finding power; it’s also fun to use.

From the upper view of this Ford 5.0 Mustang head, it is easy to conclude that the guide bosses are hardly conducive to good flow. Doing nothing more than the seemingly obvious of streamlining the guide bosses (as in the lower photo) increased rear-wheel output of a stock machine from 198 to 214 hp. The time involved was less than three hours.

At this point, you may be convinced you need a bench but the possible cost and learning how to use it may seem too daunting. Well, the good news is you do not have to spend much money here. Instead of buying a $10,000 pro-built bench, I am going to show you how to do the job for about 2 to 3 cents on the dollar.

Building a Really Trick Low-Cost Bench

May 2008 was a Vizard milestone as far as modifying engines goes. It represented, for me, 50 years of cylinder head porting and flow testing. Initially, the methods I used were crude and simple to the extreme, but what I did worked very well and produced top-notch results. Looking back on these early efforts, I realized many years later I should have continued flowing heads using my original method, instead of being swayed by convention.

Maybe you’re already familiar with the use of a flow bench. If so, you see that, once again, I do fly into the face of convention—but not without good reason.

Here, starting back in 1958, is how events unfolded. My first flow bench was in fact my mother’s vacuum cleaner. I mounted the cylinder head to be tested (an A-series head as per Austin A35 and later the Mini Cooper engine) on the block with a means of opening the valves. I then attached the suction side of the vacuum cleaner to the bore and sealed the hose so there were no leakages at this point. Next a spark plug with the middle removed and a piece of 1/4- inch-diameter copper tube glued in was installed into the spark plug hole and connected up to a manometer made from clear plastic tubing stapled onto an 8-foot-long piece of 2 x 4 wood. This was marked out in inches from 48 inches at the bottom all the way through zero to plus 48 inches at the top (note: a more powerful vacuum cleaner needs a taller manometer). The plastic tubing was looped so that the bottom of the “U” was formed about a couple of feet below the bottom of the 2 x 4. Water with food dye was used to fill the “U” section of the U-tube until it reached the zero mark (see Illustration 7-1).

As you can see, there is not much to this bench. It can be constructed in a few hours for a minimal expenditure. If you already have a shop vac, then the rest of it probably costs less than thirty bucks.

How It Works

Here is how this simple bench works: the more open the valve is, the lower the pressure drop as measured by the manometer. At low valve lift, let’s say 0.050 inch, the manometer can read (depending on the vacuum cleaner’s capability) anywhere between 60 and about 100 inches of water. If the flow at a given lift is improved, the manometer reading at that lift drops. Let’s say that the stock head at 0.050 valve lift produced a reading of 80 inches. We then do some seat blending on the valve and head casting and on the next test the manometer reading is 70 inches. This means the flow has gone up, so the pressure drop pulled by the vacuum cleaner for a first approximation, the flow has increased by the square root of 80 divided by 70. That works out to 1.069, or 6.9-percent improvement.

7-1. The type of flow bench shown here is the floating-pressure type. Round up the minimal parts required, and you can build one like this on a Saturday morning. If you already have a block and a vacuum cleaner, the rest of what is needed can usually be sourced for less than $75.

At this stage, we have a bench that shows only whether the port is better or worse. Now, here comes the aspect that separates the methodology of use with this bench from 99.99 percent of all others out there: By measuring the pressure drop to determine “better-or-worse” scenarios, we can see that it is not testing the head at a standard pressure drop as per almost every pro-built bench does.

The standard-pressure-dropmeasuring method applies a fixed amount of suction/pressure to the head as it is tested. For instance, a big Superflow bench typically uses a 28-inch pressure difference across the cylinder head. As the valve is opened, the suction drops; so the operator opens up a control valve to bring the applied pressure back up to 28 inches. To determine whether or not flow has improved requires that the results have to be calculated and converted to cubic feet per minute (CFM). This is the complex, math filled, part of flow bench design that deters most people from building their own bench.

At this point, you may be thinking that not having the ability to establish the head’s flow in CFM is a small price to be paid. The most important feature—that is, determining better or worse—has been achieved. At the end of the day, that is what really counts.

After having built my “bench,” I spent a couple of years wondering how I could get real CFM numbers off the bench like the guys at Westlake (at the time the big, if not biggest, name worldwide in airflow). Then one day I was talking to an engineer who was somewhat older than I and he happened, during our conversation, to drop the key words for me. These words were “standard pressure drop.” Well there we go; I then realized that if I could adjust the test pressure/vacuum so that it was a constant, then I could, through a big bunch of complex math, calculate the CFM. So over the next few years, I built increasingly more sophisticated benches until finally, during the early 1970s, I built a bench that conformed to British Standards as far as precision gas flow measurement was concerned. It was a highly accurate 15-foot-long monster. Cylinder head guys came from 200 miles around to get their work tested. I even did some work on it for the McLaren F1 team.

Some Rethinking

By the mid 1980s, I began to have second thoughts on the use of a standard pressure drop to test heads. The issue here is the sort of pressure differences seen between the port and cylinder in a live-running race engine. In 1984, S-A Design published Power Secrets by the unforgettable Smokey Yunick. The author goes to some lengths to show that the pressure differential needs to be above a certain value to get dyno results that sort of coincide with what you might expect from any flow increase. The number that Smokey came up with was 28 inches of water. That is about an industry standard as of 2010. But here is the problem: A race engine does not see a fixed standard pressure drop. Here, we need to take a look at what actually happens.

Although referred to as a four-cycle engine, a high-performance/race engine is actually a five-cycle engine. The fifth cycle (No. 5) is the exhaust-driven start to what subsequently becomes the regular piston-driven induction stroke (No. 1).

The induction system on a true race engine is, for the most part, exhaust driven. That is, the scavenging pulse from the tuned exhaust pulls a far bigger depression in the cylinder than the piston going down the bore. On something like a NASCAR Cup Car engine, this can amount to as much as 120 or more inches at TDC in the overlap period. The draw on the intake port at TDC can be so strong that, even though the piston is virtually parked, the intake charge in the port can be traveling into the cylinder at as much as 90 mph! When the valve is near wide open and the piston is moving at peak piston speed (this is between 72 and 74 degrees after TDC), the draw on the valve is between 15 and 20 inches of water.

All the forgoing leads us to one conclusion: If we want to more nearly simulate what happens in a running engine, our intake flow testing needs to be done at a high-pressure drop at low valve lift, and a lower one at high lift. This is exactly the way the flow test rig illustrated here functions. An uncontrolled vacuum source (such as a shop vac) pulls a large vacuum when the intake valve is closed and a progressively lesser vacuum as the intake is opened. So running a “floating” pressure drop, as we are doing here, is a more realistic simulation of actual flow during engine operation At this point, we have a flow situation that more closely mimics the pressure differentials seen in the cylinder/intake port of a running high-performance engine. So what are the advantages of testing in this manner? Let’s analyze that now.

Within the induction of an effectively exhaust-tuned engine, the negative-exhaust pulse pulls a far higher vacuum on the intake port than does the piston traveling down the bore. This high vacuum occurs at low lift, thus justifying the use of a higher flow-bench pressure drop at low lift and a lower one at high lift. The flow bench produces a floating-pressure drop.

Shown here is the port velocity of a two-valve race engine just prior to peak torque RPM. Note the intake velocity at TDC at the start of the intake is 85 feet per second. That is almost 60 mph!

If the pressure drop used is too low, the flow pattern that develops is not the same as it would be at a higher pressure drop. If we use a lowenough pressure drop, the flow will be virtually laminar. Fortunately for us, laminar flow becomes unstable and turns to a turbulent flow at pressure differentials well below whatever we are likely to see or use. Basically, laminar flow never occurs in a running engine; the flow is always turbulent.

Returning to the effects of lowtest pressure differentials, let’s say only 2 inches is used. At such a lowpressure drop, the flow in the port is slow and air stays attached even around the worst short-side turn. When a result from a test like this is corrected to, say, the commonly used 28 inches, it produces numbers that are much higher than if the head were really tested at 28 inches.

By running tests at real-world test pressure drops, we create the same pattern of flow-reducing separations as occur in actual use. Here’s an example to illustrate the situation: Take a typical aftermarket high-performance 23-degree small-block Chevy head and flow test the exhaust port at 28 inches. At this test pressure, it does not lag that far behind a megabuck NACAR Cup Car head having the same size of valve. Up to about 0.500-inch lift, the 23-degree head nearly mirrors the Cup Car head. At a little over 0.500, the Cup Car head slowly shows its superior capability. At 0.700 lift, the Cup Car head has about a 15-percent advantage over the best 23-degree heads.

Step up the test pressure to 120 inches, and the picture changes dramatically. At about 0.200 inch, the flow on the 23-degree head separates on the short side. This partially shuts off part of the port area that is normally used for the high-speed flow on the long side. With the Cup Car head, there is no real flow separation until about 0.500 lift, and what does occur is relatively minor in nature. The consequences here are that, from 0.200 on up, the Cup Cars port pulls away from the 23-degree port. And by the time 0.700 lift is reached, it is close to a 20- to 25-percent improvement.

From this, we conclude that port modifications that produce the most positive results are doing so by addressing those flow patterns and improving the port shape to best deal with them. The bottom line is this: A cheapo flow-bench test setup is actually a better tool for developing an intake port than a $10,000 commercial flow bench. The only down side at this point is knowing just how many CFM the head is flowing if each reading could be corrected to the common 28-inch pressure drop. Without this number, you won’t be able to make a comparison with other flow test results to be able to gauge your work compared to others. If you really need results in CFM, this can be fixed relatively easily as we shall see; but for now let’s consider the exhaust.

Flowing the Exhaust

Without making some rather fancy test equipment, we are not going to be able to flow the exhaust at real, live test pressures. Normally, when an exhaust valve opens, the cylinder pressure is somewhere between 90 and 120 psi. If you are intent on having a pump that develops this kind of test pressure, even for just the low-lift tests, then be aware that you need about a 200-hp motor to drive the pump. Very few flow bench setups are capable of this (though I understand from some of Ford’s engineers that their bench can approach real-world exhaust pressure drops), and that it costs a mere seven-figure number to build. For the most part, we flow the exhaust at 28 inches, and live with the fact that it is not the best way to do things. However, we find once again that our low-buck bench, with its uncontrolled floating pressure drop, actually does a better job than a commercial bench at a fixed pressure drop.

When I first wrote about this bench in my online magazine, flow bench guys bombarded me with emails, saying it would not work, could not produce accurate results, etc. It seemed so ridiculous for someone to make a comment that something can’t be done after it has been proven. And as for accurate results, given the application of a few precautions, this bench design can produce extremely accurate results and repeatability.

Establishing Repeatability and Accuracy

The most critical factor in getting usable data from a floating-test-pressure flow bench is making sure that the reading you take today will, if nothing changes on the test item, be the same tomorrow, next week, and next year. The simple way to do this is to have two or three test orifices and check that each delivers the same pressure drop it originally did when tested. If it does not, then the voltage to the motor has changed, and we know that happens just from demand from the rest of the circuit community around you. The way I stabilized voltage, back in the day, was to use a transformer to step up mains voltage by about 15 percent and then drop it to a standard voltage with a rheostat. That way, the motor voltage could be finely adjusted so that the test orifices showed the pressure drop originally assigned to them.

Quantifying Results in CFM

My friend Roger “Dr. Air” Helgesen built a bench that worked along the same lines as this in the late 1970s and still uses it to this day to flow heads and intake manifolds. As usual, Roger adopted a singularly simple way to convert the pressure drop seen on the manometer to CFM at 28 inches, using nothing more than a sheet of graph paper and a few calibration orifices. Accuracy on the order of 3 percent (that’s about the same as a typical, commercially available bench) can be had here if suitable care is taken to eliminate unwanted leakages, stabilize supply voltage, and make frequent reference checks against the calibration plates. Illustration 7-2 shows dimensions for these calibration orifices. These flow a certain amount of air when tested at 28 inches. By drawing a graph as per Chart 7-3, a curve can be developed of depression versus flow at 28 inches for your particular setup. Build a bench like this and you are in business.

7-2. These holes need to be machined to a very smooth finish. As for tolerances, make them as accurate as possible (+/- 0.001 is acceptable). The hole sizes and X–Y coordinates are: Hole 5: 0.210 Dia. on 4.30/3.40 inches. Hole 10: 0.296 Dia. on 3.95/4.10 inches. Hole 20: 0.419 Dia. on 3.05/4.30 inches. Hole 40: 0.594 Dia. on 2.10/3.80. Hole 80: 0.840 Dia. on 2.05/2.5 inches. Hole 160: 1.185 Dia. on 3.70/2.40 inches. Radius entry for all holes is 0.25 inch. This is machined such that the edge of the radius goes out to 80 degrees, not the full 90. The back of the 5- and 10-size holes must be chamfered with a 90-degree cutter. For the 5 hole, the chamfer should be 0.460 diameter, and, for the 10 hole, 0.546 diameter.

7-3. By plotting the individual orifices against the depression seen, a graph can be developed. The subsequent pressure drop with the head being tested can then be converted to CFM.

It is entirely practical to flow test a head using no more of a vacuum/ pressure source than is delivered by a typical shop vac.

Porting

Follow what I have to say here and your home porting costs will be a lot less than just going down to your local industrial tool-and-cutter supply outlet.

Up to now the subjects covered have looked at what is needed in the way of port and combustion chamber design to make the most of any particular head casting. What we now look at is the cutting, grinding, and finishing processes that are needed to produce whatever port and chambers are being reworked.

If you want to upgrade a bench, it can be done for a lot less than you may think. Here the Audie Technology Flow Quick setup is being tested. Essentially, this is a computerized, instrumented version of the floating-pressure bench detailed earlier. For full details on this bench go to www.motortecmagazine.com

Die Grinders

If you have no porting equipment already at hand, then a decision is needed on two points: cheap or expensive, and electric or air. My advice here is start cheap. As to air or electric, you need to make that decision on your own.

So we need to port like pros, yet do it on a typical hot rodder’s budget and that leads us to the next question: Do you already have air in your shop? If not, do you plan on getting an air compressor for other needs such as blowing off parts, running air tools, using a spray gun, etc.? If your shop already has air and if you have to buy a die grinder (bearing in mind that 18,000 to 20,000 rpm are needed), the best route to go is with an air-powered die grinder. The principle reason for this is that there are some really cheap die grinders that can be had for as little as $20. A decent electric die grinder that lasts any reasonable time costs five to seven times that amount. The cheaper air grinders aren’t as well made, but I have a fix that extends their life from 3 months to 3-plus years.

These days, you do not need to spend a fortune on compressors to drive an air die grinder. I bought this one in 2005 for $285, and it meets all my grinding/porting needs.

Here is what I use to lube all my air tools. As of 2010, it is the best I have found. And for the record, Boeing seems to agree with this finding.

After three years of extensive use, my previous cheap die grinder was replaced with this cheap die grinder. BND lube is the key to long life from cheap grinders.

If your air compressor does not have an easy-to-use pressure regulator, then get yourself a budget one (as seen here) to control speed.

Even if your shop does not currently have compressed air, I recommend you seriously consider the air grinder route here. If you are building engines, you need shop air for many reasons.

There is an old saying, “You only get what you pay for.” That holds good for cheap die grinders. Cheap grinders use even cheaper bearings, seals, and the like, and as a result wear out a whole lot faster. My experience with a substantial number of cheap die grinders is that most only do about four to six pairs of heads before becoming nearly unusable. However, I have found that, of all the different air tool lubes I have used, the one from BND can extend the life of a cheap grinder by a factor of ten—good for about a hundred individual heads.

With cheap die grinders, you don’t have too many choices as to the body style of the die grinder. More expensive die grinders are offered in several styles that can be categorized basically as short- or long-body. In the cheaper range, it is almost inevitably just the short style that is offered. This is okay. The longbody grinders do offer an advantage when it comes to porting intake manifolds but, for 95 percent of the uses you are likely to encounter, you can get by with a short-body grinder.

Having acquired a die grinder that goes fast enough, then you have to think about how to slow it down; not all jobs call for a speed upward of 18,000 rpm. Like die grinders, a speed control need not cost that much. A simple air-pressure regulator gets the job done.

Removing Metal—Fast

Porting is all about reshaping ports and combustion chambers to more effective forms. And 95 percent of the time this involves removing metal. If you are short on knowledge as to how best to do this, it can be tedious beyond belief!

The two materials most likely to be reworked are aluminum and cast iron. Each, to an extent, calls for a particular specification of carbide cutter. Though initially not so cheap, a $25 to $30 carbide can easily port 20 aluminum heads and be re-sharpened for about half its initial cost.

Monetarily, it looks like a straightforward deal here but the price of carbides is highly variable. The cost of a quality carbide can vary from as little as $15 a pop to as much as $35 for what is essentially the same tool. Where you purchase the item has a lot to do with the cost, and there seems no real set rule here.

For most porting jobs, a small variety of carbide cutters suffices. For aluminum, a coarse-pitch tooth form is needed, and for iron, a fine one.

A small, 50-cc bottle of the Dr. Air cutting fluid goes a long way. This is how much was left after porting four sets of heads.

Generally, I avoid professionalporting supply and tool distributors. The customers they deal with are usually machine shops employing 5 to 105 machinists who have to be kept busy or money goes out the door. But there are discount shops that, though a lot less common, can be found in bigger cities.

And there are auto swap meets. Surprisingly enough, you can often get porting supplies at such events at bargain rates, but you do need to know what you are buying and what you should be buying. Let me explain. You can purchase carbides that cut a smooth finish or you can buy carbides that remove metal at a rapid rate, and sometimes you can buy a carbide with a tooth form that is pretty good in both arenas. But would you recognize one from the other? You probably would not, because it takes an experienced eye. You can see a little problem cropping up here with cost and performance.

Fortunately, I have the solution: Purchase Dr. Air porting supplies from Dr. J’s Porting Supplies. The bottom line here is that, worldwide, there may be thousands of places you can get carbides, but there are probably only a handful in the entire world that can sell you, at significantly below-regular prices, carbides explicitly designed and produced for the porting community.

Aluminum and cast iron have radically differing properties, so it’s hardly surprising that, though there is a little overlap, each requires its own style of tooth form for optimal results. In essence, we can say that cast iron normally requires a finer tooth pitch than aluminum. Too fine a tooth pitch and aluminum clogs up the cutter in nearly no time flat. Too coarse a pitch on cast iron and the cutter dances all over the place and barely gets to grips with the job of removing metal.

As may be expected, there is a fair amount of technique and technology involved in porting. We look at a little more of what is required here but, for the most part, this subject is dealt with in detail in my porting school in online magazine at www.motortecmagazine.com.

Final Shaping and Finishing

Given the right die-grinder speed, cutters, and cutting fluid, ports and chambers can be reshaped to the point they need little work with abrasives to finish them off. However, for the “pro look,” a finishing process is needed. But let’s be clear here: A highly polished finish is, in most cases, not only unnecessary, but also detrimental to power output. A shine in the chambers is fine because it cuts heat transfer. In the ports, though, and especially in intake ports, a decent 60- to 80-grit finish gets the job done. Always keep in mind that it is shape that rules, and not shine!

Most cartridge rolls are supplied in boxes of 100, which is okay for a pro shop but is a little pricey for the home porter. These Dr. Air 50-count boxes are much more wallet friendly.

Finishing ports off requires such things as abrasive rolls, disks, and flap wheels. These all come under the heading of consumables, and they can get expensive. It may sound like I am replaying an old record here but, once again, I suggest using Dr. Air products. Depending where you might otherwise source them, Dr. Air consumable abrasives are about 40 percent cheaper. Equally important for many end users is that they are available in Bonus Boxes of 50-plus at a time. Also from the same source, you can get a starter kit, which ports more than a set of V-8 heads.

Results

If you are going to build engines, you are going to need the equipment I have described in this chapter, even if you elect not to port your own heads but buy them. Such jobs as port matching and generally cleaning up intakes, oil passages, oil return routes, and the like, call for the same tools as porting heads. If you do port your own heads, what kind of return might you expect? The answer here depends on just how good or bad the initial head casting may be. The following examples should give you at least an inkling of what might be expected.

Starting with a typical V-8, we can expect that pocket porting nets a useful gain. A 360-hp 350 with stock 186 factory head casting went up to 388 after a morning’s work on the heads. A mildly modified 2-liter non-VTEC Honda climbed from 155 hp at the wheels to 167 with pocket porting, and 170 with a basic full-porting job. A basic full-porting job on an MGB head, along with the intake cleaned up and some reworking of the twin 1-1/2-inch carbs, pushed rear-wheel horsepower from 64 to 80.

If you are starting out on your porting ventures, here is what I recommend as a starter kit. It finishes a set of V-8 heads.

These flow curves are a result of the pocket porting shown earlier in the chapter. A 16-hp increase was realized for a minimal cost; plus, three hour’s works represents a good return on investment of time and money.

These days, small-block V-8 head castings from EQ, RHS, and Dart are very “porter” friendly. Even a beginner of reasonable dexterity can produce a professional-looking and -performing end product.

Thirty hours of basic porting on this small-block Chevy EQ head produced very positive results. The flow produced was sufficient to top 600 hp on a 383-ci engine with a 12:1 CR.

Do you feel inclined to port your own heads? If not, CNC-ported heads from a serious, professional porting shop are an option, but at a cost. Shown here are the high-flowing 227-cc port Chevy heads from Dart.

This Ultra Pro Machining CNC-ported LS6 Chevy head not only had more flow but the intake swirl was also increased over stock. With a minimal cut out of the intake to keep velocity up, this head improved output at low RPM while delivering more than 40 hp over stock.

Edelbrock offer this CNC-ported Victor head for the small-block Ford. My dyno tests with this head have shown stellar results, especially with stroker motors.

On most four-valve heads, the port dividers are rounded at the leading edge because a cast sharp edge is not very practical to do. By knife-edging the divider as seen here, measurable extra flow can be found for minimal effort.

Here is the chamber of a race-winning four-valve head I did in about 2005. The thermal-barrier coating is plain to see. What is not so obvious is that this is a polyquad design using four different valve sizes to promote swirl and cut cross flow from intake to exhaust during the overlap.

Knife-edging the trailing edge of the divider on the exhaust is not a good move. Target a rounded form (as seen here) for best results.

Fully porting heads for two-valve applications can really pay off. A small-block Chevy that made 404 hp on stock factory 492 heads delivered 482 hp on a set of fully ported Dart 200-cc-port aluminum heads. This is what happens when intake flow goes up from 184 cfm at full valve lift to 303 cfm, and exhaust from 124 to 210 cfm. A 2.0-liter Pinto with a stock head made 130 hp and, with an all-out head, 152 hp.

Heads are not the only component we can port. A clean-up and a port matching job on a small-block Chevy Edelbrock Super Victor only took about 3 hours of work, and boosted power from 556 to 571 hp on an engine of 383 cubes. Can’t afford an intake? A 10-hour porting job on a stock Q-Jet small-block Chevy intake bumped output from a near-stock 350 from 282 to 304 hp.

Written by David Vizard and Posted with Permission of CarTechBooks

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